Issue #10565: Iterator ABC should require both __next__ and __iter__.

Lib/_abcoll.py

@@ -90,7 +90,8 @@ class Iterator(Iterable):
     @classmethod
     def __subclasshook__(cls, C):
         if cls is Iterator:
-            if any("__next__" in B.__dict__ for B in C.__mro__):
+            if (any("__next__" in B.__dict__ for B in C.__mro__) and
+                any("__iter__" in B.__dict__ for B in C.__mro__)):
                 return True
         return NotImplemented
 

Lib/test/test_collections.py

@@ -356,8 +356,14 @@ class TestOneTrickPonyABCs(ABCTestCase):
         for x in samples:
             self.assertIsInstance(x, Iterator)
             self.assertTrue(issubclass(type(x), Iterator), repr(type(x)))
-        self.validate_abstract_methods(Iterator, '__next__')
-        self.validate_isinstance(Iterator, '__next__')
+        self.validate_abstract_methods(Iterator, '__next__', '__iter__')
+
+        # Issue 10565
+        class NextOnly:
+            def __next__(self):
+                yield 1
+                raise StopIteration
+        self.assertNotIsInstance(NextOnly(), Iterator)
 
     def test_Sized(self):
         non_samples = [None, 42, 3.14, 1j,

Misc/NEWS

@@ -43,6 +43,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #10565: The collections.Iterator ABC now checks for both
+  __iter__ and __next__.
+
 - Issue #10242: Fixed implementation of unittest.ItemsEqual and gave it
   a new more informative name, unittest.CountEqual.